1. Field of the Invention
The present invention relates to a colour display device using optical valves to produce an image by the projection, on a display means, of modulated and recombined beams of coloured light.
2. Description of the Prior Art
Present-day systems of this type generally have a source of white light, the light flux of which is separated into three beams of coloured light (for example, red, green and blue). The light from the three beams is then polarised, and then directed towards the three crystal matrices, each one of which modulates the beam that it receives as a function of control signals. The three images that are formed on the matrices are then enlarged and projected simultaneously by one or more lenses on the final display means of the image, namely for example on a screen, a wall etc.
One of the problems raised by presently used systems is their light efficiency: the maximum light flux emitted by the source has to be recovered so that the final image may be as luminous as possible and, at the same time, it must be seen to it that its homogeneity in colour and in luminance is not impaired. To this end, it is necessary for the illumination at any point of one of the matrices to be equal to that of the corresponding point on each of the other matrices, so as to avoid any colour variation in the final image. It is known that this condition may be achieved easily in provided that the distances between the source and the three matrices are made equal.
Another problem is that of achieving the maximum reduction in the space occupied by the systems for the recombination and projection of the three coloured images when a single objective is used to project them and to enlarge them. Indeed, in this case, after the three beams have each crossed a matrix, they should be recombined to form a colour image. Minimizing the space occupied by the recombination and projection systems amounts to reducing the distances, which besides are equal, from each of the matrices to the projection lens. The recombination means that enable this consist of two plane surfaces processed so as to intersect at their middle at right angles, in forming a substantially cube-shaped geometrical FIGURE, whence the name "recombiner cube" which shall be applied hereinafter to these means. The three matrices are then positioned so as to be before three faces of the cube parallel to the intersection of the processed surfaces, so that the recombined light goes through the fourth of these faces before which the projection lens is placed.
This kind of an arrangement of recombination and projection systems is found in a projection device disclosed by the Japanese patents Nos. 62-254589 and 62-22219, filed on behalf of SEIKO and shown in FIG. 1 of the appended drawings. In this projection device, the white light coming from the source S is divided, by means of two dichroic mirrors D1, D2 placed in parallel, into three beams, coloured red, green and blue, which are respectively directed towards the optical valves V1, V2 and V3, which are positioned in the above-described manner with respect to a recombiner cube C and an optical projection device 0. The drawing clearly shows that the distances from the source S to the valves V1, V2 and V3 are not equal, the distance up to the valve V1 being substantially longer.
The same remark is applicable to the projection system described in the published patent application No. EP-A-0 331 326, filed on behalf of MATSUSHITA. This projection system has a device for the separation of the beams formed by the two dichroic mirrors D'1, D'2 that intersect each other perpendicularly at their middle as illustrated in FIG. 2 (this device shall hereinafter be called a "divider cube" by analogy with the above-mentioned "recombiner cube"). Here, the equal paths of the light between the source S and the valves V1 and V2 are very substantially longer than the path up to the valve V2.
By contrast, in another projection system known from the patent application No. EP-A-87201558.1 now EP-A-028927, filed on behalf of PHILIPS and shown in FIG. 3, where the divider device includes two parallel dichroic mirrors D1, D2, the distances between the source S and the three valves V1, V2 and V3 are quite identical. But then, this condition dictates a position of the valves that does not enable the minimizing of the distance from the three valves to the projection lens 0 by using a recombiner cube.